1. Field of the Invention
The present invention relates in general to a toner composition. More specifically, the present invention relates to a toner composition having an external additive with specific components that are present in a specific ratio, featuring stable charge distribution of the toner in a developer of an electrophotographic image forming apparatus, improved toner supply capability, prevention of filming and a fog, and high quality images.
2. Description of the Related Art
In recent years, electrophotographic image forming apparatuses such as laser printers, fax machines, and copiers have been widely used to obtain the benefit of high-speed operation and high quality images thereof. Depending on which kind of developer is used, the electrophotographic image forming apparatuses may be divided into a dry type and a wet type. Particularly, the embodiments of the present invention are related to a dry developer.
FIG. 1 is a schematic diagram of a conventional dry electrophotographic image forming apparatus based on a non-contact developing system. In the operational process of the dry electrophotographic image forming apparatus of FIG. 1, a photosensitive object (or photosensitive drum) 100 is charged by a charging device 600, and an image is exposed to a laser beam from a laser scanning unit 900 to develop a latent image on the surface of the photosensitive drum 100. A dry toner 400 in a supply roller 300 is supplied to a developing roller 200. A toner layer regulating device 500 ensures that the toner supplied to the developing roller 200 has a thin and uniform thickness. During this process the toner is frictionally charged by the contact between the developing roller 200 and the toner layer regulating device 500. Particularly, M/A and Q/M of the toner transferring to a developing area are adjusted. M/A is a measurement of weight of the toner per unit area (mg/cm2), and Q/M is a measurement of amount of charge per unit weight of the toner (μC/g). Both M/A and Q/M of the toner are measured on the developing roller after the toner had passed through the toner layer regulating device. The toner, having passed through the regulating device 500, is then developed to an electrostatic latent image on the photosensitive drum 100, is transferred by a transfer roller (not shown) to a recording medium, and is fused by a fuser (not shown). Any residual toner on the photosensitive drum 100 is wiped by a cleaning blade 700, stored in the residual toner collecting bin 800, and the printing process is repeated again starting from the charging step to the image forming step.
In general, a dry toner contains a colorant, a binder, a control charge agent, a releasing agent, and optionally other additives to meet the functional requirements of the toner. The additives are divided into an internal additive that is added into toner particles, and an external additive that is added to the surface of the toner particles. Although the toner comprises particles of several micrometers that form a print image on the recording medium, chargeability and fluidity of the toner play a major factor in determining the quality of the print image. Therefore, various kinds of compounds are present in the toner composition as external additives to provide the toner with effective fluidity, charge stability (or chargeability), and cleanability.
A noncontact and nonmagnetic one-component developing system is regarded as very advantageous because the system features a—minimized size, facilitated color correspondence, effective gradation, and high-resolution image quality. The most important feature in the noncontact, nonmagnetic one-component developing system is ensuring that the charge and charge distribution of the toner remain constant after repeated print operation, that is, are substantially the same as in an initial printing operation. In this way, stable developing capacity may be maintained, and fogging and filming may be prevented.
The most effective way to provide the toner with a uniform charge is generally the formation of a thin toner layer on the developing roller. However, when the layer is made thin, it consequently imparts substantial stress to the toner and deteriorates the toner itself. In addition, when the thin toner layer is formed on the developing roller, the charge of the toner is increased, and this, in turn, lowers the developing efficiency and the image density.